PO2 Max - Why 1.4/1.6?

[Jim Lapenta]

Historically, I believe it was the US Navy that originally came up with the limits, but I'm sure there was other research going on throughout the world. If you can find an old US Navy Dive Manual or look through some history books you can find more of the background. If I recall the US Navy determined that very physically fit Navy divers could tolerate exposures up to PPO2 of 2.0 for short duration of time during military operations. These limits were then accepted by commercial divers along with recommendations for OTU limits. Later Dr. Bill Hamilton and Dick Rutkowski developed the first "recreational" nitrox limits and made them more conservative. This might have been in combination with NOAA at the time?? Here's a descent write up on the topic of setting the limits: http://www.swiss-cave-diving.ch/PDF-dateien/Oxygen-Hamilton.pdf

There's lots of good books that cover some of the history of this too. "The Terrible Hours" about the Sqaulus submarine rescue that took place back in the 1930's is good read. It talks about how US Navy Dive Officer Charles "Swede" Momsen came up with some of the limits for deep diving and mix gas to rescue the trapped crew members. I don't recall if covers O2 specifically. There's also lots of good books written about Dr. Hamilton and Rutkowski and early days of tech diving. I think the old NAUI and IANTD manuals covered some of this, but maybe it is glossed over in more the modern nitrox training.

In the end, this is still similar to asking someone how many beers they can handle. Everyone is different and their physiology changes from day to day. Unfortunately, you can't test everyone for their individual susceptibility or account for the environmental stresses they will experience on each unique dive. So the best that is currently available is to recommend conservatism since O2 toxicity and convulsions are very often fatal under water. When I started cave diving it was very common to run 1.4 PPO2 as standard bottom mix. Unfortunately, good people have been lost using the limits and now it is much more accepted to run a 1.2 PPO2. I strongly recommend using 1.2 or below for working or longer duration dives.

I started using 1.3 for the working portion of dives back in 2007-2008 during my NAUI tech training. One of the guys we used to see on the boat in Lake Erie died in a cave of oxtox running a po2 of a little over 1.3. It became policy after that to lower our PO2 for the dives we were doing in cold water with heavy workloads.

 

[Akimbo]

Some readers may find the discussion in this thread helpful: Oxygen Toxicity Limits & Symptoms

There are a lot of factors that influences the onset of Oxygen toxicity symptoms, not the least of which is human variability. Choosing a "safe" PPO2 limit comes down to probabilities, not absolute certainty. The same is true of decompression models. I don't think that the difference of 1/10th of one percent in PPO2 provides much certainty, nor is there an absolute on-off switch between safe and unsafe.

 

[100days-a-year]

Some readers may find the discussion in this thread helpful: Oxygen Toxicity Limits & Symptoms

There are a lot of factors that influences the onset of Oxygen toxicity symptoms, not the least of which is human variability. Choosing a "safe" PPO2 limit comes down to probabilities, not absolute certainty. The same is true of decompression models. I don't think that the difference of 1/10th of one percent in PPO2 provides much certainty, nor is there an absolute on-off switch between safe and unsafe.

I think people just like absolutes in science. Human bodies underwater don't lend themselves to constants and that is difficult for some folks.

 

[tursiops]

I think people just like absolutes in science. Human bodies underwater don't lend themselves to constants and that is difficult for some folks.

Yeah, people like things to be black and white. Maybe only white, in fact. Disregard all other.

 

[Akimbo]

I think people just like absolutes in science. Human bodies underwater don't lend themselves to constants and that is difficult for some folks.

Agreed. Unfortunately that also makes people extremely uncomfortable, bordering on fearful, when they see "out of range" exceptions to rules they perceive as hard and fast. Another problem, IMO, is a lot of diving fatalities are publicly blamed on OxTox when there is no scientific evidence. A possible or contributing cause of death is not THE cause of death.

As a technology optimist, I have no doubt that one day we will have a complete understanding of OxTox AND have the instrumentation required to monitor all the variables that influence it, or directly monitor the biochemistry of OxTox. Unfortunately, none exist today.

Until then, understanding the limits of our knowledge and the current guidelines is the best we can do. I like to think that ScubaBoard helps.

 

[-hh]

Here in Italy training of new divers was done using CC pure-oxygen rebreathers since 1948 and until around 1980. These units were called ARO, and were developed during WW2 by Italian Submarine Incursors (COMSUBIN), which managed to use them in a number of successful attacks to UK vessels.
The problem of oxygen toxicity was deeply studied by military doctors in the forties, but these results were not made available to the public.
I was trained in 1975, so I was using this ARO equipment for several months (an OW course was 6-months long at the time). At that time the military information had just been released, so our diving manual did contain a "safety curve" table, defining the maximum exposure time at each ppO2, based on those medical studies.
This did allow to go down to 18m, but for just a few minutes.
My OW certification enables me explicitly to dive with an ARO down to 10m (where ppO2 is almost 2.0 bar) for 30 minutes, with a surface interval breathing air for no less than 15 minutes before diving again.
But there was a second point very relevant: when using an ARO in open water (not in the pool) the usage of a full-face mask was mandatory. This was very important, as in case of convulsions a full face mask avoids to loose the mouthpiece and to drawn. After a couple of minutes of convulsions, the diver comes back to control, and if he has not drowned he can usually ascend safely.
So these "safety limits" were not "safe enough" for avoiding convulsions at all, they were set for military operations with CC rebreathers, and allowing that under some circumstances the diver will be hit by CNS toxicity.
When the problem of oxygen toxicity was moved from CC pure-oxygen military-grade rebreathers to recreational OC Nitrox systems, it was necessary to avoid entirely the risk of convulsions (also because the usage of a full face mask is generally not practiced for recreational diving).

Some useful trivia for you:

The UK "Charioteers" were using rebreathers that weren't pure O2 and thus, had a slightly better OxTox tolerance.

The book "Above us the Waves" was published in the 1950s before this was declassified, but if you read that book with this knowledge, you can see where some of the statements alluded to something being different without divulging just what.

Hence the max ppO2 limits were lowered initially to 1.6 bar, and more recently to 1.4 bar.
I think that a 1.6 bar limit is reasonable for using a CC rebreather or a deco stage tank at small depth, where
the mixture has an high percentage of oxygen. And instead, when doing a deep dive around 40m, better to stay with a safer 1.4 bar limit, as there are other adverse factors which can exacerbate the problem, such as CO2 retention, narcosis, effect of high nitrogen pressure, etc.
Personally I avoid Nitrox when there is chance to extend the dive below 30m depth. In those cases I prefer to use plain air with a shorter dive time, or, even better, to plan for a deco dive (with the required equipment, training and logistic setup), which I consider generally safer than a dive executed "on the edge of the NDL", without being equipped for a deco dive.

My general take on it is that the risk curve starts to really bend upwards at 1.6 PPO2 with the significantly shorter allowed single dive duration. As gear technology (e.g., rebreathers) and DCS algorithms got digitized, it became much more common for divers to approach the 1.6's time limit (IIRC 45 minutes), along with those who "went over", either on time or on PPO2, which invariably would have resulted in some "whoa, not as safe as we thought". Add a pinch of lawyers and advocating for lower risk rates than what was acceptable years earlier and it makes sense to see the number get backed off. Some of this parallels what we've seen in standard recreational dive tables too: the old "100ft for 25min" isn't a no-deco/no-stop profile anymore...

 

[Angelo Farina]

Some useful trivia for you:
The UK "Charioteers" were using rebreathers that weren't pure O2 and thus, had a slightly better OxTox tolerance.

Very interesting..
I found some info here:
British commando frogmen - Wikipedia
But it does not says much about the rebreathers used. The picture shows an old Davis apparatus, which is a pure-oxygen rebreather on my knowledge. In the text it says that some "Charioteers" were using German oxygen systems captured from German aircrafts, possibly some Drager apparatus, but I did not find much info on them.
It also says that many suffered of OxTox problems and one died during training.
Also interesting that, after 8 September 1943, the Italian frogmen did merge with the UK frogmen, and conducted successful actions against Germany, for example in La Spezia. I wonder if they did manage to bring to UK also the Italian ARO systems....

 

[tursiops]

Very interesting..
I found some info here:
British commando frogmen - Wikipedia
But it does not says much about the rebreathers used. The picture shows an old Davis apparatus, which is a pure-oxygen rebreather on my knowledge. In the text it says that some "Charioteers" were using German oxygen systems captured from German aircrafts, possibly some Drager apparatus, but I did not find much info on them.
It also says that many suffered of OxTox problems and one died during training.
Also interesting that, after 8 September 1943, the Italian frogmen did merge with the UK frogmen, and conducted successful actions against Germany, for example in La Spezia. I wonder if they did manage to bring to UK also the Italian ARO systems....

When I lived and worked in La Spezia, 1968-1972, my neighbor was Roberto Frassetto, one of the Italian frogmen who use the O2 rebreathers to bomb the British ships in Malta in 1941. The stories he told me don't quite match the Wikipedia article on him...

 

[Akimbo]

The UK "Charioteers" were using rebreathers that weren't pure O2 and thus, had a slightly better OxTox tolerance.

I wonder how that worked in those days? The only option in WWII was all-mechanical semi-closed circuit rebreathers. That would be pretty risky for combat swimmers approaching ships in a harbor. Even a rebreather using 90% Nitrox would require periotic or continuous venting to prevent anoxia.

I understand that the PPO2 limit in the Royal Navy was 3.0 at the beginning of the war and then dropped to 2.0 after more testing. See Oxygen Toxicity Limits & Symptoms, Post #5

Related Story:
My very first direct experience around rebreathers was with a friend who build one. He had a PhD in chemistry and was normally a pretty meticulous guy. It was about 1964 and most of the parts were from war surplus pure O2 rebreathers. He was testing it in a swimming pool and forgot to purge the loop of air. After a few minutes be breathed the Oxygen down enough that he slowly blacked out, but the single breathing bag was still inflated enough that adding O2 was not triggered — which was manual as I recall. Nobody suspected it until he rolled on his back at the bottom of the pool and the mouthpiece floated out.

I was 13. Two of the adults dove in, recovered him, and started CPR. He came to and said "sh*t, I forgot to purge the bag". It made quite an impression.

Years later, I was on a team headed by a saturation-qualified US Navy Master Diver investigating an Oxygen fire at the UDT and SEAL (they were still separate at that point) training facility in San Diego (Coronado). We observed how they purged their pure O2 rigs as part of their predive prep. They blow a LOT of O2 threw the loop and closed the mouthpiece valve before disconnecting from the fill station. It was about 4x as much gas as I would have guessed would be required.

 

[Twoblues]

I just wrapped up the online portion of my nitrox course a few days ago, and have been reading around here and elsewhere since, trying to gain a deeper understanding of things. One thing I haven't been able to find an explanation on: why was I taught to use a max PO2 of 1.4, or 1.6 for contingency planning? I understand the concept of what PO2 is, and I understand the risks/ramifications of exceeding these numbers, so I'm not asking "why can't I use 1.8, or 3.5?" I'm trying to get my head around why these are the numbers, from a theoretical perspective. I'll be amazed if this info isn't already here somewhere, but I'm not sure how to refine my search terms to get beyond conversations about which number to use, rather than why those numbers are the right ones to use.

Operationally, I'm completely OK "just accepting" 1.2/1.4/1.6 are acceptably safe and well-validated numbers. I have no interest in debating the choice of these numbers, and I'm certainly not trying to suggest there's a better choice. Intellectually, I just want to know more. Were these numbers arrived at simply through mass trial and error, were they backed out of decompression algorithms, or are they based on existing physiological research independent of the rec/tech dive world? I can imagine plenty of scenarios where human oxygen toxicity limits might be of interest to other sectors like aerospace, medicine, commercial diving, etc.

I've found plenty of good "books to read" lists here and elsewhere, and I've got a copy of Deco for Divers on order to start with. Unfortunately it looks like it'll arrive a couple days after my next stint out of town, so I'll have to read something else on the plane. Is there maybe a better book on this topic I should earmark for the next one on the list?

Do you know Diver's Alert Network? Their experts often have discussions on the science of EAN diving